The syndrome of heart failure is a common course for the progression of many forms of heart disease. Heart failure may be considered to be the condition in which an abnormality of cardiac function is responsible for the inability of the heart to pump blood at a rate commensurate with the requirements of the metabolizing tissues, or can do so only at an abnormally elevated filling pressure. There are many specific disease processes that can lead to heart failure. Typically these processes result in dilatation of the left ventricular chamber. Etiologies that can lead to this form of failure include idiopathic, valvular, viral, and ischemic cardiomyopathies.
The process of ventricular dilatation is generally the result of chronic volume overload or specific damage to the myocardium. In a normal heart that is exposed to long term increased cardiac output requirements, for example, that of an athlete, there is an adaptive process of slight ventricular dilation and muscle myocyte hypertrophy. In this way, the heart fully compensates for the increased cardiac output requirements. With damage to the myocardium or chronic volume overload, however, there are increased requirements put on the contracting myocardium to such a level that this compensated state is never achieved and the heart continues to dilate.
The basic problem with a large dilated left ventricle is that there is a significant increase in wall tension and/or stress both during diastolic filling and during systolic contraction. In a normal heart, the adaptation of muscle hypertrophy (thickening) and ventricular dilatation maintain a fairly constant wall tension for systolic contraction. However, in a failing heart, the ongoing dilatation is greater than the hypertrophy and the result is a rising wall tension requirement for systolic contraction. This is felt to be an ongoing insult to the muscle myocyte resulting in further muscle damage. The increase in wall stress is also true for diastolic filling. Additionally, because of the lack of cardiac output, there is generally a rise in ventricular filling pressure from several physiologic mechanisms. Moreover, in diastole there is both a diameter increase and a pressure increase over normal, both contributing to higher wall stress levels. The increase in diastolic wall stress is felt to be the primary contributor to ongoing dilatation of the chamber.
Prior treatments for heart failure associated with such dilatation fall into three general categories. The first being pharmacological, for example, diuretics and ACE inhibitors. The second being assist systems, for example, pumps. Finally, surgical treatments have been experimented with, which are described in more detail below.
With respect to pharmacological treatments, diuretics have been used to reduce the workload of the heart by reducing blood volume and preload. Clinically, preload is defined in several ways including left ventricular end diastolic pressure (LVEDP), or indirectly by left ventricular end diastolic volume (LVEDV). Physiologically, the preferred definition is the length of stretch of the sarcomere at end diastole. Diuretics reduce extra cellular fluid which builds in congestive heart failure patients increasing preload conditions. Nitrates, arteriolar vasodilators, angiotensin converting enzyme (ACE) inhibitors have been used to treat heart failure through the reduction of cardiac workload by reducing afterload. Afterload may be defined as the tension or stress required in the wall of the ventricle during ejection. Inotropes function to increase cardiac output by increasing the force and speed of cardiac muscle contraction. These drug therapies offer some beneficial effects but do not stop the progression of the disease.
Assist devices include mechanical pumps. Mechanical pumps reduce the load on the heart by performing all or part of the pumping function normally done by the heart. Currently, mechanical pumps are used to sustain the patient while a donor heart for transplantation becomes available for the patient.
There are at least four surgical procedures for treatment of heart failure associated with dilatation: 1) heart transplantation; 2) dynamic cardiomyoplasty; 3) the Batista partial left ventriculectomy; and 4) the Jatene and Dor procedures for ischemic cardiomyopathy, discussed in more detail below. Heart transplantation has serious limitations including restricted availability of organs and adverse effects of immunosuppressive therapies required following heart transplantation. Cardiomyoplasty involves wrapping the heart with skeletal muscle and electrically stimulating the muscle to contract synchronously with the heart in order to help the pumping function of the heart. The Batista partial left ventriculectomy surgically remodels the left ventricle by removing a segment of the muscular wall. This procedure reduces the diameter of the dilated heart, which in turn reduces the loading of the heart. However, this extremely invasive procedure reduces muscle mass of the heart.
One form of heart failure, ischemic cardiomyopathy, results from the formation of one or more zones of ischemia, or infarction, of the myocardium. Infarction occurs when blood supply to the heart tissue has been obstructed resulting in a region of tissue that loses its ability to contract (referred to as infarcted tissue). The presence of infarcted tissue may lead to three conditions in the heart causing cardiac malfunction. These conditions are ventricular aneurysms (ventricular dyskinesia), non-aneurysmal ischemic or infarcted myocardium (ventricular akinesia), and mitral regurgitation.
Ventricular aneurysms typically result from a transmural myocardial infarction, frequently due to the occlusion of the left anterior descending artery (LAD). This results in a transmural infarcted region of the apical portion of the left ventricle and anterior septal. A ventricular aneurysm is formed when the infarction weakens the heart wall to such an extent that the tissue stretches and thins, causing the left ventricular wall to expand during systole (dyskinesia). FIG. 55 illustrates a ventricular aneurysm A occurring in the apical region of left ventricle LV. As shown by the shaded region in FIG. 55, aneurysm A includes infarcted tissue 24 that results in a reduced wall thickness when compared to adjacent non-infarcted wall regions, as shown by the unshaded regions in FIG. 55. FIG. 55 also shows the septal wall S partially infarcted, again shown by the shaded region. The ventricular aneurysm also may be dyskinetic, meaning that when the ventricle contracts, the aneurysm further dilates, or bulges, outward. The infarcted region of the septal wall S also may be particularly dyskinetic, especially in the case of the infarcted tissue having progressed to an aneurysm.
The bulge resulting from an aneurysm can have several serious effects on the heart and its performance that can lead to in both morbidity and mortality. For example, because the bulge creates a geometric abnormality as well as a region of non-contracting tissue, thrombosis is more likely to occur in that region. Thrombosis is the formation of a blood clot, or thrombus, that can cause other medical complications, such as a stroke. An ischemic stroke is a blockage of blood flow to the brain that occurs when the thrombus breaks free and is ejected out of the ventricle.
Another serious effect this bulging can have is the denigration of the heart's pumping function. The aneurysmal bulge creates problems with pumping function in at least three ways. First, the infarcted tissue does not contribute to the pumping of the ventricle because it does not contract (akinesia). To account for this loss of pumping, remaining portions of the ventricle wall may contract more to maintain cardiac output. If the infarcted region thins and progresses to an aneurysm (dyskinesia), this effect is further exacerbated by the aneurysm expanding with a portion of the blood from the ventricular contraction. This further increases the contractile requirement of the remaining functional myocardium.
Second, the aneurysmal bulge alters the geometry of the entire ventricular chamber. Thus the ventricle develops a larger radius of curvature, which directly applies more tension to the heart wall, as characterized by LaPlace's law.
Third, over time, the above two conditions lead the functional muscle of the ventricle to work harder than normal. This can lead to continued dilatation of the ventricle, increasing tension in the walls of the heart, with increased myocardial oxygen requirement and further progressing heart failure.
Non-aneurysmal ischemic or infarcted myocardium (akinesia) occurs when a major coronary artery is occluded and results in infarction in the myocardial tissue, but without a bulging aneurysm. In a manner similar to an aneurysm, the akinetic ischemic or infarcted zone ceases to participate in the ventricular contraction. This results in the functioning, contractile myocardium needing to contract more to make up for the lack of contraction of the akinetic zone. Typically, the result is the entire ventricle increasing in size, which increases wall stress. Again, since the functioning myocardium must work harder, continuing progression of heart failure can occur.
Mitral regurgitation also may result from infarcted tissue, depending on the region of the ventricle that has become infarcted or aneurysmal and any subsequent overall ventricular dilation. Mitral regurgitation is a condition whereby blood leaks through the mitral valve due to an improper positioning of the valve structures that causes it not to close entirely. If the infarcted or aneurysmal region is located in the vicinity of the mitral valve, geometric abnormalities may cause the mitral valve to alter its normal position and dimension, and may lead to annular dilatation and the development of mitral regurgitation.
Typical treatments of infarcted tissue, and ventricular aneurysms in particular, include a variety of open surgical procedures. In the case of a ventricular aneurysm, traditionally, a “linear” aneurysmectomy is performed. This procedure involves the removal of aneurysmal portions of the anterior wall along with any thrombus that may exist. FIG. 41 illustrates the result of a conventional surgical method when an aneurysm occurs in the distal left ventricle. According to this method, the region of aneurysmal scar tissue that extends through the entire thickness of the chamber wall (transmural infarction) is removed by incision and the remaining border zone regions 24′ (i.e., regions where infarcted tissue meets non-infarcted muscle) are sewn together with a suture 27. In a linear aneurysm repair procedure, the ventricular septal wall S that is infarcted is left untouched. Additionally, the septal wall generally remains untouched because simple excision and suturing does not involve excluding or cutting the septal wall. Usually, only those wall portions having infarcted tissue through their thickness (transmural infarcted) are removed while the portions having infarcted tissue only on an inner wall (endocardial infarcted) are left in place. The term border zone refers to this region of endocardial infarction. This surgical procedure results in some infarcted tissue regions remaining in the heart chamber, particularly any infarcted tissue in the septal wall. The effects of the remaining non-contractile tissue stresses the remaining contractile tissue because this contractile tissue must “make up” for the non-contracting and often dyskinetic tissue. Over time, these effects can continue to lead to progression of heart failure.
These procedures have to be performed with the patient on cardiopulmonary bypass. The heart also may be stopped in order to perform the surgery. Any thrombus inside the ventricle is removed. Clinical results of this traditional surgical procedure have been mixed with respect to improvement in cardiac function.
Newer surgical approaches include the “Dor” and “Jatene” procedures. In the “Dor” procedure, the aneurysm is removed and an endocardial patch is placed to cover the dyskinetic septal wall portion of the aneurysm. In this manner, at least the portion of stroke volume “lost” to dyskinesia is restored. In the “Jatene” technique, a purse string suture is placed at the base of the aneurysm. The infarcted septal wall is circumferentially reduced by inbrication with sutures. The result is that most of the aneurysmal tissue is excluded from the ventricle. These procedures address the infarcted septal wall, generally left untouched in the traditional linear aneurysmectomy, by either exclusion or by the use of a surgical patch. These newer techniques are also used in cases of non-aneurysmal infarctions (akinesia). In these cases, the exclusion or elimination of the infarcted region reduces the size and therefore the radius of the chamber, thereby lowering wall stress.
These various described techniques for treating infarcted and aneurysmal tissue regions in the heart wall suffer from limitations and drawbacks. For instance, many of the surgical techniques involve invasive incisions in the heart wall which can be traumatic and risky to patients. Also, while these procedures attempt to improve cardiac function by removal of the aneurysm or infarcted tissue, they only minimally reduce the wall stress of the remaining contractile ventricle. Furthermore, patients typically undergo cardiopulmonary bypass and/or their heart is stopped during many of these surgeries.